1. Field of the Invention
The invention relates to sound reproduction equipment and method for reducing the level of acoustical reflections in a room.
Methods concerned in the invention are used in conjunction with electrical systems intended for producing sound including sound reproduction equipment or electronic music instruments in order to attenuate acoustic reflections in a space into which the sound is being reproduced. Such a space may be, e.g., a room arranged for listening music or monitoring sound quality.
2. Description of the Related Art
In the prior art, undesired acoustic reflections and room resonances have been attenuated by generating such a cancelling sound wave that via the destructive interference of acoustic waves attenuates the unwanted sonic pressure wave components. The cancelling wave to an original sound wave is acoustic energy incident at the same frequency and at least essentially out-of-phase with the original sound wave. In turn, the amplitude of the cancelling wave determines the degree of sound attenuation. In order to achieve perfect cancellation of the original sound, the cancelling wave must have a frequency and amplitude exactly equal to those of the original sound and a phase exactly opposite to that of the original sound at a given spatial point. If the undesired sound is composed of a plurality of frequencies, the above-listed cancellation criteria must be fulfilled separately for each frequency component of the sound to be cancelled. This technique is described in U.S. Pat. No. 2,043,416, for instance.
When generating a cancelling sound wave, it is necessary to know the properties of the sound to be attenuated with a reasonable accuracy in order to produce the required cancelling sound signal in a proper manner. Conventionally, this is accomplished through, e.g., measuring the sound to be cancelled by a microphone, processing the measured signal in order to produce the required cancelling sound signal and converting the processed signal into a physical cancelling sound by a loudspeaker mounted at the desired point of cancellation. The placement of the microphone in respect to the loudspeaker in the direction of sound propagation has been dictated by the selected cancellation technique depending on whether the so-called feedforward or the so-called feedback method is used.
In the feedforward method, the microphone has been located in front of the loudspeaker in the direction of propagation of the sound to be cancelled, at a point permitting the microphone to measure the sound to be cancelled alone, without being responsive to the cancelling sound wave. The measured signal has been processed in respect to the signal delay in the sound cancellation equipment and the signal transfer function plus the acoustic propagation delay of the sound to be attenuated between the microphone and the loudspeaker radiating the cancelling sound wave. In a practicable system, there is further needed a second microphone located after the loudspeaker in the direction of the original sound propagation, whereby the signal of the second microphone is used for monitoring the efficiency of sound cancellation and for controlling the signal level to be fed to the loudspeaker. The feedforward-type generation of the cancelling sound wave is described in U.S. Pat. No. 4,122,303, for instance.
In the feedback method, the microphone is located after the loudspeaker in the direction of propagation of the sound to be cancelled, whereby the microphone is responsive to both the loudspeaker radiating the original sound and the loudspeaker radiating the cancelling sound wave. The goal herein generally is to minimize the amplitude of the signal measured by the microphone or at least to bring it down to a desired level. If also the microphone is located after the loudspeaker in the direction of propagation of the sound to be cancelled, the method must be capable of predicting the level of the signal to be attenuated on the basis of the measured interference signal. To attain a good attenuation efficiency, a number of different methods of processing the measured signal have been developed. A more detailed description of the cancellation signal processing technique can be found in U.S. Pat. No. 4,878,188, for instance. The prior art also includes cancellation sound generation techniques based on combinations of feedforward and feedback methods.
In U.S. Pat. No. 4,899,387 is further disclosed an apparatus for cancelling low-frequency acoustic resonances in a room used as an acoustic space. The apparatus is particularly suited for improving room acoustics in listening to music. The major single factor causing acoustic frequency response variations typically is the listening room itself that may readily cause deviations as large as 20 dB at some frequency in the amplitude response in a given point of the listening room. These deviations are caused by the interference of sonic pressure waves reflecting from the walls of the listening room with pressure waves radiated directly from the loudspeakers. Obviously, the need for improved listening room acoustics is urgent.
The embodiment described in cited U.S. Pat. No. 4,899,387 attempts to solve the above-described problem by placing cancellation apparatus units in the room at the pressure maxima or in the immediate vicinity thereof. Said cancellation apparatuses comprise a microphone for sensing the sound pressure waves and signal processing means and a cancelling loudspeaker for producing the cancelling pressure waves to the reflected original sound thus measured. In this arrangement, the microphone is located close to the cancelling loudspeaker, and with the help of a feedback circuit, the goal is to alter the acoustic impedance of the cancelling loudspeaker such that the effect of the room acoustics on the smoothness of the sound field is eliminated. This technique bears the risk of instability of the feedback loop that also includes the sound cancellation apparatus itself, whereby the system may start to oscillate.
In Pat. Appl. No. JP 6-62499 is disclosed another system for eliminating reflected pressure waves. Differently from those described above, this arrangement uses no microphones placed in the listening room, but rather the signal is sampled directly from the stereophonic audio system used for producing the original audio signal. The system disclosed in cited publication JP 6-62499 comprises cancelling loudspeakers placed in the listening room and a cancellation signal generator adapted to feed said loudspeakers. The cancellation signal generator itself includes delay and amplitude control circuits for delaying the signals of the left and right audio channels and for setting the signal amplitudes separately for each cancelling loudspeaker. The cancellation signal generator further includes summer circuits for combining the signals processed in the delay and amplitude control circuits into output signals to be taken to each of the cancelling loudspeakers and inverter circuits for inverting the phase of each combined signal. The delay circuits are controlled to delay each signal individually by the time of sound propagation from the original sound loudspeaker to the cancelling loudspeaker. E.g., in a system of four cancelling loudspeakers, the signals for each loudspeaker are formed from the signals of both the left and the right channel with appropriate delays. Additionally, also the signals of the loudspeakers and/or cancelling loudspeakers reflected from the walls can be taken into account, whereby a different delay and gain must be set for each signal separately.
A problem in the apparatus of cited publication JP 6-62499 is that, in spite of the extremely complicated technique of cancellation signal generation, the system is incapable of eliminating all the reflections occurring in the listening room and particularly not the diffraction waves caused by obstacles in the listening room. It must also be noted that the point-source type cancellation sound radiators used according to the publication even theoretically can eliminate reflected pressure waves but only in very singular points of the listening room. Other points of the room remain void of any attenuation, but rather the apparatus brings about a greater level of distortion and unwanted fields of reflected sound in the listening room. At some locations of the listening room, the original sound and the cancelling pressure wave are even in phase, whereby interference wave thus formed amplifies the reflection wave almost two-fold in amplitude in regard to the initial reflection already when using one cancellation signal.
It is an object of the present invention to provide an entirely novel method that in a simple manner is capable of reducing the disturbing low-frequency acoustic reflections otherwise occurring in a listening room without the disadvantage of simultaneously creating new disturbance at higher frequencies. It is another object of the invention to provide a sound reproduction apparatus offering attenuation of annoying reflected sound in a listening room with a simpler construction and improved performance over that available in conventional apparatuses.
The goal of the invention is achieved by way of sampling the sound signal electrically from the equipment used for producing the original sound such as the amplifier feeding a loudspeaker. The sample signal is processed in order to generate the cancellation signal for attenuating the pressure level of reflections and the cancellation signal thus formed is converted by means of at least one loudspeaker into a cancelling sound in the listening room where the impact of reflections is desired to be attenuated. According to the invention, the sample signal used for generating the cancellation signal is filtered so as to free the signal from essentially all the sound components that can create a so-called diffuse sound field in the listening room. The term diffuse sound field refers to a situation in which the sound does not excite a pressure pattern of systematic reflections in the listening room, but rather the sound field is comprised of an incoherent composite waves formed by reflections from a plurality of different surfaces and/or diffraction waves about different objects. In such a diffuse sound field, each point of the listening room receives waves related to the same original sound but received as a sum of partial waves in different phase and incoherently directed, which makes the behaviour of the interfering wave pattern difficult to predict. The filtration is advantageously performed before the delaying operation and the setting of the signal phase and amplitude. According to the invention, per each cancellation signal feeding the respective cancellation sound radiator is set a single optimized delay that minimizes the disturbing resonance in the room. However, differently from the approach of cited publication JP 6-62499, the suitable delay is not determined directly from the times of propagation of the acoustic signals by computational means, but instead, the delays required in a specific acoustic system are optimized by on-site tests. Also the level of the cancellation signal is advantageously determined on the basis of measurements performed on the attenuation results in the listening room, and the goal is to set the level of the cancellation signal to such a value that achieves the desired change in the acoustic pressure wave pattern of the room at the frequencies to be attenuated without causing an excessive attenuation to the original acoustic signal. Thus, the object of the invention does not include a complete elimination of the undesired pressure waves to be attenuated at the points of controlled cancellation.
The invention offers significant benefits.
As the sample signal is taken in electrical form from the sound-producing system, the method or apparatus according to the invention disposes of the need for any microphones in its operation. Resultingly, the construction of the sound reproduction apparatus may be simplified and it can operate without being dependent on such external factors as any possible changes in the response of a monitoring microphone caused by aging or heating, for instance. Furthermore, since the cancelling sound is produced according to the invention only at low frequencies, the application of the invention does not introduce any superfluous high-frequency signals of disturbing nature in the listening room.
As compared to methods and apparatuses of the feedback type, a further benefit of the invention is the sound-reproduction apparatus according to the invention has no electroacoustic feedback circuit, whereby it involves no risk of an unstable state of oscillation.
A further benefit resulting from low-pass filtration prior to taking the signal to the delay circuit is that the information content of the signal to be delayed is reduced and the delay can be accomplished by means of a relatively simple digital circuit. Hence, the invention makes it possible to manufacture a simple apparatus at a low cost for improving the acoustics of a listening room.
A still further benefit of the invention is that the low-frequency spectrum of the cancelling sound allows the arrangement according to the invention to be implemented using only one delay circuit per each cancellation sound radiator. Hence, it is also unnecessary to define separately the delays for the left and right channels in a stereophonic system. Broadly, even a multichannel sound reproduction system can manage in a similar manner with only one delay circuit per each cancellation sound radiator.
A still further benefit of the invention is that when the volume of the cancelling sound is set appropriately below the level of the original sound, the level of the low-frequency bass sound whose reflections are to be attenuated can be retained unchanged in the acoustic space, which is contrary to the behaviour of conventional techniques in which the goal is to produce a cancelling sound capable of completely nulling the unwanted frequency component of the original sound field.
When the delay and gain of the cancellation signal to be launched in the listening room is set in a proper ratio to the sound emitted by original signal sources, the invention can by the above-described means achieve a consistent cancellation result irrespective of changes made in the acoustic environment of the listening room. As the apparatus needs no sound-sensing microphone during sound reproduction, there is no risk of having function of the apparatus according to the invention affected by vibration or other disturbance normally imposed on a microphone in a sonic field.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and of the scope of the invention will become apparent to those skilled in the art from this detailed description.